Within data communications, two major standards dominate. One is CCITT X.25, the international standard for data communication by way of a public data packet network. The other is Systems Network Architecture (SNA), IBM's protocol, which has become a de facto standard, at least in North America, by virtue of widespread use.
Supporting communications between SNA computers and SNA terminals on a packet switching network has passed through several phases, each of which has compromised either the SNA functions, the packet switching functions or both. Early implementations required each terminal cluster controller, and hence all of the terminals connected thereto, to be connected to a single computer. Thus, the switching capability of the packet network was not used beyond merely establishing the virtual circuit to a predetermined computer. A later implementation allowed terminals to establish communications with any one of a number of computers available from the packet network; however, as these terminals no longer belonged to any one computer important SNA management services were not supported.